The main objective of the study is to analyze the process by which transcobalamin II (TC II) promotes the penetration of vitamin B-12 into cells. For the coming grant year, the cell model will be the HeLa cell grown in monolayers. The uptake sequence in this model is known to include a penetration of TC II-B12 into the cell, the participation of a carrier protein now known only as the intracellular binder, and the release into the culture media of several carrier proteins not yet fully identified. First there will be an identification of the participating carrier substances using as reference standards known proteins of transport of vitamin B12. Secondly, there will be an identification of the form of vitamin B-12 carried by these substances. Finally, it will be determined whether transcobalamin II is essential in the initiation of the process or whether the small amount of free vitamin B-12 that can enter a cell is sufficient. BIBLIOGRAPHIC REFERENCES: Hall, C.A. Perfluorocarbon Emulsion in the Perfusion of Canine Organs. Federation Proceedings 34:1513-14, 1975. Hall, C.A., Rappazzo, M.E. & Green, P.D. Release of Transcobalamin II by Canine Organs. Proc. Soc. Exp. Biol. & Med. 148:1202, 1975.